A decrease in the air pressure of tires may degrade the driving performance and comfort of automobiles. Therefore, it is desirable that the driver recognize a decrease in tire air pressure to take the necessary measures. Accordingly, apparatuses for warning drivers of abnormalities in tire air pressure have been proposed. A typical tire air pressure warning apparatus includes a tire air pressure monitoring device. The warning apparatus sends out a warning when the air pressure of a monitored tire becomes lower than a predetermined value. Japanese Patent No. 2639856 describes one type of warning apparatus.
A monitoring device employed in the prior art is illustrated in FIG. 5. As shown in the drawing, the monitoring device is provided with a transmitter 7, which is secured to the inner side of a wheel rim 1, and a valve stem 10 through which air flows when charging the assoicated tire with air. The tranmitter 7 includes a case 2, which houses a pressure detector 3, a signal processing circuit 4, and a battery 5. The pressure detector 3 detects the air pressure of the tire. The detected air pressure is converted to electric signals and transmitted to a receiver, which is installed inside the automobile, by the signal processing circuit 4. The battery 5 energizes the signal processing circuit 4. The monitoring device 7 further includes a base 13, which is connected to the signal processing circuit 4 by a wire 6. The valve stem 10 functions as an antenna for transmitting tire air pressure signals.
The valve stem 10 and the case 2 are insert molded and formed integrally with each other. The base 13 is also formed integrally with the case 2.
An annular groove 14 extends along the surface of the base 13 to receive an O-ring 15. The valve stem 10 is fastened to the wheel rim 1 by a washer 8 and a nut 9. The washer 8 and the nut 9 also serve to seal the wheel rim 1. The wire 6 extending from the signal processing circuit 4 is soldered to the valve stem 10 to electrically connect the signal processing circuit 4 to the valve stem 10.
Under the transmission theory of radio waves, it is known that the transmission efficiency is highest when the impedance at the output side of a circuit matches the impedance at the input side of the antenna. Therefore, if the impedance of the signal processing circuit 4, which functions as the output side, does not match the impedance of the transmission antenna, which functions as the input side, the signals produced by the processing circuit 4 cannot be efficiently radiated as radio waves.
The valve stem 10 and the metal wheel rim 1 are in conductive contact with each other. Thus, impedance irregularity takes place in the transmission antenna. This results in a matching loss, or impedance mismatch, between the impedance of the signal processing circuit 4 and the impedance of the transmission circuit. As a result, the matching loss weakens the radio waves radiated from the transmission antenna. In some cases, the radio waves may not reach the receiver, which is located in the passenger compartment.